Fractional Slot Stator Windings ? 4

[edison123]

We have a 3.3 KV, 1490 RPM (4 pole), 1000 HP Motor with 84 stator slots, which we want to convert to 990 RPM (6 pole) motor. For 6 pole, we get a fractional slot winding (84/3x6=4 2/3 slots per pole per phase). Can any one suggest the actual arrangement of coil groups around the stator periphery that will be electrically and magnetically balanced for 6 poles? For example, I claim the arrangement 555 455 544 555 455 544 is the best. But, our consultant claims the arrangement 545 545 464 545 545 464 is electrically better ? Views from any experienced ac machine designer will be appreciated.

 

[electricpete]

Well, I put that formula into excel. edison's most recent configuration gives
Magnitude 26.72 26.64 26.72
Phase 36.71 -83.57 156.15
which seems pretty darned close.

The consultant's recommendation gives
Magnitude 23.25 23.35 26.72
Phase 32.14 -78.34 156.15
which is quite a bit off.

I wonder if I'm doing that right. I'll try to post the spreadsheets.

 

[electricpete]

OK - it is uploaded.

http://www.geocities.com/pschimpf/fractionalslot.htm

Do you guys agree this is a reasonable way to evaluate the balance? edison - why did you mention a requirement for "magnetic balance" of 180 degrees opposite pole bands?

 

[electricpete]

I keep on talking about "induced" voltages. It comes from the terminology of Liwschitz/Garak. It is as if we treat it as a 6-pole syncronous generator... then look to ensure that the induced voltages add up to create a balanced three-phase set.

 

[edison123]

pete,

You have to have magnetic pole balance (same no. of amp turns in opposite poles) to prevent uneven magnetic forces. i am sure you'll agree that if one pole has say 5 coils and the exactly opposite (180 deg mechanical) has only 4 coils, then you will have uneven rotating magnetic forces, which will definitely produce vibrations and other undesirable harmonics.

i am yet to go thru' your upload. will get back once i have a look at it.

 

[electricpete]

thanks edison. That makes sense.

I made an error on the consultant file. I changed the A,B,C column, but forgot to change the polarity column. When I change the polarity column it gives me identical results to your's (edison's).
Magnitude 26.72 26.64 26.72
Phase 36.71 -83.57 156.15

Spooky similarity. They look close enough to balanced that the error might be round-off error. Still not sure if I'm doing it right.

 

[edison123]

pete,

does your xls files have some formula links since i have got only blanks in your angle calculation columns ? may be you can mail me yr results directly.

 

[GusD]

Hello edison123

Edison,you have a lot of good information regarding your future motor modification .It would be interesting to do some load testing on this drive after the mods, and compare new motor efficiency/performance versus old one.
If you can provide that information it would be great.

Thanks a lot.

GusD

 

[edison123]

GusD,

We are right now into the project. Expect to commission the motor in another 6 weeks. Will surely post the results in this thread.

 

[electricpete]

Hi edison. I don't know if you are still interested in looking at those files. I would be intersted to hear your opinion.

I looked at those files and they still read ok on my computer. I think there are differences in how excel is set up. It needs to have the functions for handling complex functions. Maybe you can try reading on another computer.

One other alternative is that I can convert all the results to values (vs functions) and e-mail it to you. Unfortunately I lost your address. e-mail me if you want me to send it like that.

 

[jbartos]

Suggestion: Although the machine with 84 slot 6 pole 4 2/3 slots per pole per phase is not included in Referece:
1. M. G. Say "Alternating Current Machines," John Wiley & Sons, 1978, page 88 "Fractional - Slot Winding" section, the 16 pole 3 phase machine with 108 slots and 60deg phase spread with 3 3/5 slots per pole per phase is included.

 

[edison123]

epete,

my e-mail : eewemf@myrealbox.com

Just now, I rechecked your XLS file. Still not working. Would appreciate if you can mail your program.


jbartos,

Thanks for your reference which I have already checked.


To all interested in this post:

The conversion to 6 poles is under progress. In all likelihood, the converted motor will be ready for testing by first week of august.

 

[edison123]

An Update to all interested in this post.

Finally, the motor, redesigned and rewound for 6 pole operation, was successfully test run today on no-load with low voltage supply. (Actual load test in its own bed will be done in about 3 weeks). Here are the low voltage test results:

Original parameters for 4 pole : Stator volts 3.3 KV, stator current 162 A speed 1490 RPM (4 pole) with rotor volts of 910 V and rotor current of 500 Amps.

Redesigned parameters for 6 pole : Stator volts 3.3 KV, stator current 115 A speed 990 RPM (6 pole) with rotor volts 660 V and rotor current of 475 Amps.

Rotor was injected with 450 V and induced stator volts was 2150 volts. The motor ran smoothly at 1020 RPM (the supply frequency was 51.3 Hz from a separate DG). The no-load rotor current at 450 V was 166 amps. (I know, the no-load current is a bit high but then the requirement was for a high pull-out torque necessiating higher magnetic loading).

No electrical vibration (due to fractional slot stator winding in the new 6 pole design) and low mechanical vibration. Max velocity was only 1.8 mm/sec pk.

Thanks electricpete, GusD, teco and jbartos for sharing your valuable opinions with me. Your ideas meant a lot to me.

 

[electricpete]

Thanks for sharing that.

 

[GusD]

edison123

Thanks for sharing the good news about your motor mod.
"All is well that ends well"

Regards
GusD'

GusD